Update Quickstart with Jupyter Hub

[nrichers]

In this PR:

• In the Quickstart, update the Explore ValidMind topic to replace Google Colab with Jupyter Hub as the preferred way to get started.
• Simplify the Explore ValidMind topic (make steps to run all notebook cells in first section optional, remove "Steps" headings, text edits)
• Move the existing Google Colab steps into a new tutorial in parallel with the existing Docker one
• Clean up the Get started topic to remove extraneous links
• Update the Before you begin topic with additional domain info and Jupyter Hub access
• Add What's next links for topics in the Quickstart that flow into each other (when you reach the bottom of a topic, you now have a link that takes you to the next one)
• Move a number of follow-on links that take you to the Guide section and the FAQ into Next steps where should have been all along
• Edits, minor improvements, etc.

Preview is available on the docs demo site. The main updates are in these topics:

External Release Notes

Test description 2 for documentation label.

[mehdi0501]

There is a typo in the title of the first paragraph (says "Juypter" instead of "Jupyter").

Otherwise, looks good!

[cachafla]

👍

[nrichers]

There is a typo

Fixed! And how embarrassing.